The present invention relates to liquid heaters having a burner to dispense hot combustion or flue gases which heat a finned heat exchange vessel filled with fluid, the flue gas being diverted about and through baffles to increase efficiency. More particularly the liquid is first preheated in a first stage in an outer jacket which is also exposed to the flue gases. Such a heater is applicable to hydronic heating systems and domestic water heating.
Hydronic heating systems circulate hot water in a closed system comprising a water heater and a plurality of radiators. Sometimes consumable hot water is also obtained through heat exchange with the closed hydronic system.
Today, the most common of domestic water heaters comprise a pressure vessel having a cylindrical wall, a hemispherical top and a concave, hemispherical bottom which is directly exposed to a gas or oil burner. The effective heat exchange surface is substantially limited to the hemispherical base. The vessel also has a central flue for discharge of flue gases and some recapture of the heat from the hot flue gases. A cool water inlet is located near the base of the vessel. The water in the vessel is heated and the resulting hot water rises to the top of the vessel for extraction on demand. The vessel is insulated along its cylindrical portion to reduce heat loss during standby periods. The efficiency of such a hot water vessel is not particularly high.
In systems having a larger heat demand, such as those used for heating living space, it is conventional to use boilers and heat exchanger furnaces which utilize large surface heat transfer areas by providing a plurality of tubes either through which or around which combustion gases pass for delivering up their heat to the heat transfer fluid on the opposing side of the tubes. Tubes are often linearly extending between opposing heads or are coiled to minimize space and maximize surface area. There are many connective joints, relatively fragile materials of construction and many opportunities for failure and resulting expensive repairs.
In the past and out of favor today due to low efficiencies, a water heater was introduced which utilized a ribbed, inverted cone-shaped water reservoir which was enclosed in an outer cylindrical casing. Such a heater is specifically set forth in Canadian patents 405,431 in 1942 and CA 473,394 in 1952, both to Wenger. An annular plenum having an upwardly diminishing cross-sectional area was formed between the conical reservoir and the casing through which flue gases were conducted for heating the reservoir. As in typical hot water heaters, cool water was introduced at the base of the reservoir and hot water was removed from the top of the reservoir. The reservoir was ribbed and heat transfer occurred substantially through conduction of heat to the reservoir from the hot flue gases passing in a co-current flow upwardly through the plenum to the reservoir""s sidewall. Hot flue gases were vented from the plenum. While successful due to their simplicity and reliability, their efficiencies became unacceptable, and eventually their use diminished. The use of coiled tubing boilers is associated with high cost and expensive repairs but have relatively high efficiencies. The cone type heaters of Wenger were inexpensive, associated with low maintenance but have only low efficiencies. These disadvantages of the prior art systems are believed to be resolved by the water heater of the present invention.
In one aspect of the present invention, a heater is provided for supplying hot water in a heating system. The heater comprises a combination of a low-maintenance, enhanced-surface area heat transfer vessel which is situated in an annular hot flue gas plenum. In a preferred arrangement using a supplemental and first stage dual-wall heating jacket, efficiency is increased so as to be comparable to more sophisticated, expensive and higher maintenance systems of the prior art. Hot flue gas flowing through the plenum is directed circumferentially by one or more perforated ring plates for enhanced convective heat transfer about the vessel.
In a broad aspect of the invention, the heater comprises: a housing having a base and an upper exhaust end for forming a plenum which conducts a flow of hot flue gas from a burner positioned adjacent the housing""s base; a heat transfer vessel having a substantially conical body with a closed tip and a closed top, the body residing substantially coaxially within the plenum so as to form an annular space therebetween through which hot flue gases upwardly flow to the exhaust end, the tip of the body being oriented closest to the burner and having side walls diverging upwardly towards the plenum""s exhaust end; an inlet adjacent the vessel top and a vessel outlet adjacent the vessel tip so that the liquid flows downwardly and countercurrent to the hot flue gas and is heated before being discharged from the vessel; and one or more annular plates located transverse across the annular space for at least partially distributing the hot flue gas about the vessel as they pass upwardly by the one or more annular plates. It is preferred to insulate the housing for this embodiment, the housing quickly achieving flue gas temperatures.
Preferably, the annular plates contain a plurality of openings therethrough, at least some of which are louvered forming baffles for urging the flue gas to circulate about the vessel. Where two or more plates are used, the baffles can be oriented in the same circumferential direction or in alternately opposite directions.
More preferably, the heater can be fitted with a preheating jacket containing the liquid for preheating it before directing to the vessel. The jacket accepts even more heat from the hot flue gas and results serendipitously in a lower outside jacket temperature which may not even require thermal insulation in when the feed liquid enters the heater at ambient temperatures. The annular jacket comprises inner and outer walls which are closed at a lower and an upper end and forming an annular cross-sectional space therebetween, the inner wall forming the housing and which is in heat conductive communication with the hot flue gases in the plenum; an inlet at the jacket""s lower end and an outlet at the jacket""s upper end so that liquid can flow from the inlet to the outlet and be preheated before discharge into the vessel""s inlet.
In another broad aspect, the preheater jacket can be combined with any of a variety of heat exchanger for convenient and more effective use of the hot flue gases. Further improvement in efficiency can be obtained by adding one or more annular plates.